1. Field of the Invention
The present invention relates to a new diffusion joining method used for joining Cu or Cu alloy contacting surfaces at a low temperature in the field of metallurgical technology and electronic engineering, which may be applied particularly to a method for preparing a multilayer printed wiring board and also relates to a paste used in the method.
2. Description of the prior Art
Hitherto, in order to join at least two contacting metal surfaces there has been used widely a joining method, such as soldering. In the electronic engineering, there has been sometimes used an ultrasonic joining method, but in general the reliability of electric connection could not be obtained by any other method than the soldering method.
However, in the soldering method a contacting metal component such as lead or tin different from copper of the surfaces to be connected sometimes causes generation of corrosion in the joint and also causes formation of intermetallic compound, which causes the problems of a faulty joint in which cracks easily generate by repeating stress. The aforemention ultrasonic joining method does not produce the above problems, but causes the problem of only a small connecting area which can not be applied to the usual metal joint.
Therefore, a first object of the present invention is to provide a new method for joining the contacting of Cu or Cu alloy surfaces which is superior in the joint reliability.
Further, many kinds of resin dispersive type conductive pastes have been used for connection of electronic parts or circuits and formation of electroconductive paths. However, this causes a problem the joint has a relatively high resistance and thus tends to indicate non-ohmic characteristic by which the resistance increases in a low current range, so that use of the conventional conductive paste is limited.
Therefore, a second object of the present invention is to provide a new conductive paste which can bring the above joining method into practice.
On the other hand, the methods used for connecting via holes of a multilayer printed wiring board in the electronic circuits are classified into two groups. One is a method for forming a conductive path in the via hole by means of electroplating, the other being a method for filling the via holes with the conductive paste which is referred to as Ag through hole method.
However, the electroplating method can provide the via hole superior in conductivity and reliability while it requires many processes and facilities for after-treating electroplating or washing drainage, which disadvantageously is costly. The other method can provide an easy-making and inexpensive Ag through hole while it has a relatively high resistance and thus tends to indicate non-ohmic characteristic by which the resistance increases in a low current range, so that use of the conventional conductive paste is limited.
Therefore, a third object of the present invention is to provide a method for preparing a multilayer printed wiring board at a low cost and having a low resistance and a high reliability.